<h2>DESCRIPTION</h2>

<p><em>i.evapo.pm</em>, given the vegetation height (hc), humidity (RU), 
wind speed at two meters height (WS), temperature (T), digital terrain model (DEM), 
and net radiation (NSR) raster input maps, 
calculates the potential evapotranspiration map (EPo).

<p>Optionally the user can activate a flag (-z) 
that allows him setting to zero all of the negative evapotranspiration cells; 
in fact these negative values motivated by the condensation of the air water 
vapour content, are sometime undesired because they can produce  computational 
problems. The usage of the flag -n detect that the module is run in night hours 
and the appropriate soil heat flux is calculated.

<p>The algorithm implements well known approaches: the hourly 
Penman-Monteith method as presented in Allen et al. (1998) for land 
surfaces and the Penman method (Penman, 1948) for water surfaces.

<p>Land and water surfaces are idenfyied by Vh:
<ul>
<li> where Vh gt 0 vegetation is present and evapotranspiration is calculated;
<li> where Vh = 0 bare ground is present and evapotranspiration is calculated;
<li> where Vh lt 0 water surface is present and evaporation is calculated.
</ul>

<p>For more details on the algorithms see [1,2,3].

<h2>NOTES</h2>

<p>Net solar radiation map in MJ/(m2*h) can be computed from the combination of the r.sun , 
run in mode 1, and the <em>r.mapcalc</em> commands.

<p>The sum of the three radiation components outputted by r.sun (beam, diffuse, and reflected) 
multiplied by the Wh to Mj conversion factor (0.0036) and optionally by a 
clear sky factor [0-1] allows the generation of a map to be used as 
an NSR input for the <em>i.evapo.PM</em> command.
<p>
Example:
<div class="code"><pre>
r.sun -s elevin=dem aspin=aspect slopein=slope lin=2 albedo=alb_Mar \
      incidout=out beam_rad=beam diff_rad=diffuse refl_rad=reflected \
      day=73 time=13:00 dist=100;
r.mapcalc "NSR = 0.0036 * (beam + diffuse + reflected)"
</pre></div>


<h2>SEE ALSO</h2>

The <a href="http://istgis.ist.supsi.ch:8001/geomatica/">HydroFOSS</a> 
project at IST-SUPSI (Institute of Earth Sciences - University school of 
applied science for the Southern Switzerland)
<br>
<em>
<a href="i.evapo.mh.html">i.evapo.mh</a>,
<a href="i.evapo.time.html">i.evapo.time</a>,
<a href="r.sun.html">r.sun</a>,
<a href="r.mapcalc.html">r.mapcalc</a>
</em>

<h2>AUTHORS</h2>
  
<p>Original version of program: The <a href="http://istgis.ist.supsi.ch:8001/geomatica/index.php?id=1">HydroFOSS</a> project, 2006, IST-SUPSI. (http://istgis.ist.supsi.ch:8001/geomatica/index.php?id=1)
<i>
<br>Massimiliano Cannata, Scuola Universitaria Professionale della Svizzera Italiana - Istituto Scienze della Terra
<br>Maria A. Brovelli, Politecnico di Milano - Polo regionale di Como
</i>
        
<p>Contact: <a href="mailto:massimiliano.cannata@supsi.ch">Massimiliano Cannata</a>


<h2>REFERENCES</h2>

  <p>[1] Cannata M., 2006. <a href="http://istgis.ist.supsi.ch:8001/geomatica/index.php?id=1">
  GIS embedded approach for Free & Open Source Hydrological Modelling</a>. PhD thesis, Department of Geodesy and Geomatics, Polytechnic of Milan, Italy.

  <p>[2] Allen, R.G., L.S. Pereira, D. Raes, and M. Smith. 1998. 
  Crop Evapotranspiration: Guidelines for computing crop water requirements. 
  Irrigation and Drainage Paper 56, Food and Agriculture Organization of the 
  United Nations, Rome, pp. 300

  <p>[3] Penman, H. L. 1948. Natural evaporation from open water, 
  bare soil and grass. Proc. Roy. Soc. London, A193, pp. 120-146. 

<!--
<p>
<i>Last changed: $Date$</i>
-->
